Designed and printed by New Zealand Post, the commemorative "stamp" is actually a pair - with the prince on one, his bride on the other and a perforated line down the middle. Which means Mr and Mrs can be torn apart.
On the unusual Niue stamp, the second-in-line to the throne is also worth $1 more than his bride - with Kate's side worth $2.40, and her husband-to-be $3.40. The stamp is priced in New Zealand dollars, the currency used by Niue.
Editor of Gibbons Stamp Monthly Hugh Jefferies told British newspaper The Telegraph the stamp was "embarrassing" and had not been designed well.
However NZ Post Stamps and Coins general manager Ivor Masters told TV3 that the design had been approved by Queen Elizabeth II, who is the nation's head of state.
"Design of the royal wedding was particularly innovative," he said. "If you think of how many of those stamps would actually be used for postage, the real focus is on collecting memorabilia."
The design was not the only controversy affecting stamps produced by NZ Post for the April 29 royal wedding, with a presentation pack of New Zealand's own stamps containing an error regarding to Prince William's birth date, The Dominion Post reported Monday.
Mr Masters said that about 3000 presentation packs with the mistake were sold, and only two customers had asked for it to be replaced. Another 300 reprinted packs with the correct date have already been bought or ordered, he said.